To measure the flow rate of a fluid to be measured flowing through a flow tube, there are used a vortex flow meter and a thermal flow meter.
As is well known in the art, the vortex flow meter utilizes the fact that, when a vortex generator is arranged in a fluid flow, within a predetermined Reynolds number range, the number of Karman vortexes generated from the vortex generator per unit time (vortex frequency) is in proportion to the flow rate regardless of whether the fluid is gas or liquid; this constant of proportionality is called Strouhal number. As a vortex detector, there are exemplified a thermal sensor, a distortion sensor, a photo sensor, a pressure sensor, and an ultrasonic sensor; these are capable of detecting thermal change, lift change, etc. due to a vortex. The vortex flow meter is a simple flow meter capable of measuring flow rate without being affected by the physical properties of the fluid to be measured, and is widely used for flow rate measurement of gas and fluid (for example, see JP 2869054 B).
A thermal flow meter is equipped with a temperature-sensitive sensor (fluid temperature detection sensor) and a heating temperature-sensitive sensor (heating side temperature sensor), and is controlled such that the temperature of the heating temperature-sensitive sensor (flow velocity sensor (heater)), which is endowed with the functions of a temperature sensor and a heating sensor, exhibits a fixed temperature difference with respect to the temperature as measured by the temperature-sensitive sensor. This is due to the fact that the quantity of heat taken away from the heater when the fluid to be measured is passed is related to the mass flow rate, which is calculated from the heating power amount with respect to the heater (see, for example, JP 2004-12220 A).
JP 2006-29966 A discloses a technique related to a multi-vortex flow meter equipped with both the function of a vortex flow meter and the function of a thermal flow meter. The multi-vortex flow meter is capable of accurately measuring from minute flow rate to large flow rate, which makes it especially superior to other flow meters.